Despite robust effects on depression and anxiety, access to mindfulness-based interventions is limited. The delivery of mindfulness training via mobile health is a promising approach for increasing access to optimized mindfulness interventions. However, for this work to move forward, it is vital to determine the necessary dose of mindfulness practice. The proposed project will begin to address this knowledge gap by evaluating an existing mobile health mindfulness intervention within a depressed/anxious sample and characterizing the association between dosage and outcomes. This Career Development Award will provide the candidate training in mobile health clinical trials and evaluation of intervention effects through ecological momentary assessment and intensive longitudinal data analysis. This new training will be leveraged to test the dosage-outcome relationship within a mobile health context. A sample of 90 individuals with elevated symptoms of depression and/or anxiety will be recruited from the community and provided with a smartphone-based mindfulness intervention providing instruction in practices central to established mindfulness-based therapies (e.g., mindfulness-based cognitive therapy). Participants will be randomly assigned to two dosages of mindfulness practice and followed for 4- weeks. Depression and anxiety symptoms and purported mechanisms (decentering, repetitive negative thinking, mindfulness) will be measured via ecological momentary assessments. Aim 1 will assess the feasibility, acceptability, and trends in changes in outcomes for the 4-week mobile health intervention using symptom measures and metrics drawn from implementation science. Aim 2 will establish the effect of experimental manipulation on mindfulness practice dosage. The ability to manipulate dosage is important for future studies seeking to determine an optimal dose of mindfulness practice. Aim 3 will characterize the association between practice dosage with symptoms and candidate mechanisms using pre-post and intensive longitudinal ecological momentary assessment data. Dynamic structural equation models will evaluate bidirectional cross-lagged relationships between dosage with symptoms and mechanisms. These models will clarify the dosage-outcome relationship and will provide estimates of effect size and heterogeneity for planning a future large-scale randomized controlled trial. All aims will be supported by didactic, experiential, and mentored training in the fundamentals of clinical research through the NIH-funded Institute for Clinical and Translational Research (ICTR) and the Center for Healthy Minds. Aim 3 will be facilitated by new training in advanced longitudinal data collection and analysis, supported by a training team with extensive expertise in these domains. Collectively, the research and training aims of this project seek to promote the development of acceptable and effective mindfulness-based mobile health interventions for depression and anxiety, providing new training in mobile health clinical trials and advanced quantitative methods. This award will enable the candidate to conduct future intervention studies aimed at reducing the burden of depression and anxiety through optimized mobile health technology.